Oil is commonly produced from a subterranean formation through a string of tubing extending to the surface. The oil is commqnly lifted through the tubing by the action of a downhole pump actuated by a sucker rod string which is reciprocated at surface by a pump jack.
A sucker rod string is made up of a multiplicity of individual sucker rods threaded together end to end to form the string. Each sucker rod comprises a main stem, typically having a length of about 30 feet and a diameter of 3/4 inches. At one end, the sucker rod has an enlarged coupling member having an externally threaded pin; at its outer end, it has a second enlarged coupling member having an internally threaded box. These coupling members typically have a diameter of 17/8 inches The tubing in which the rod string moves typically has an inside diameter of 21/2 inches.
The use of sucker rod strings in directional or slanted wellbores has been attended with problems. The steel rod string tends to lie along the low side of the tubing string and, with time, reciprocation of the rod string tends to wear away the wall of the tubing string.
This problem has been comprehended and grappled with in the prior art. One concept which has been explored involved incorporating centralizing anti-friction means into the rod string, to space the rods from the tubing and reduce wear of the tubing wall. Patents which exemplify the prior art solutions based on this concept are: U.S. Pat. No. 2,198,720 (Edgecomb et al); U.S. Pat. No. 1,508,845 (Giles); U.S. Pat. No. 2,783,028 (Jamison); U.S. Pat. No. 2,601,478 (Weir); U.S. Pat. No. 1,541,791 (Christofferson); and U.S. Pat. No. 1,507,972 (Loop).
The present invention grew out of a series of different embodiments which were built and tried.
The first embodiment involved cutting a circumferential slot in the surface of each rod string coupling. This slot was filled with a wrapping of rawhide. The rawhide had two desirable qualities: it was slippery and, if it separated from the coupling and dropped down the tubing, it did not cause damage (as would be the case with steel `junk`). However the rawhide would swell and restrict clearance needed for the movement of oil between the rod and tubing strings.
The next version tried involved using a split ring plastic sleeve in place of the rawhide. But these sleeves came off easily and dropped down the annulus between the strings and interfered with the proper operation of the rod string and downhole pump.
Consideration was then given to mounting small, protruding wheels in the coupling body. The concept considered was to cut slots partway into the body, in the manner shown by Edgecomb, and mount the wheels on pin axles. However this approach was discarded for the following reasons:
the wheels and axles would be small in dimension, due to space limitations, and thus would be liable to fail and drop out of the slot;
and, because these components would be made of steel, such failure would result in steel junk dropping to the base of the tubing string onto the pump, which is undesirable.
Another problem, which was making itself apparent during this design and testing stage, had to do with the nature of the produced oil stream being dealt with. The couplings were being used in slanted, shallow wells producing heavy oil containing much sand. Parrafin wax is commonly associated with such heavy oil. The combination of wax and sand would readily clog up moving parts.